Alan Smithee wrote:
By transfer function you mean a curve or like
the transfer function in the print dialogue?
Yes, due to historical technical reasons many RGB spaces are non-linear, namely they have transfer function that has the form of a power function, it is also called as gamma. Gamma 2.5 is the transfer function of the CRT monitors. In Mac systems the image data is by default adjusted by the operation system by gamma 1/1.45 so the effective gamma of Mac systems with CRT monitor is 2.5*(1/1.45)=1.72 but due to a fault in a historical tool (The Mac gamma tool) it was said to be 1.8. The very same fault (by the very same author) was there in the Gamma tool that the Windows version of Photoshop had up to v
4.0.1, therefore the gamma in default PC systems is said to be 2.2
while it in reality is gamma 2.5 since Windows operation systems by default do not adjust the data that goes to monitor at all.
sadRGB (sRGB) has a transfer function that is not a gamma gamma function, this has the effect that there will be more color error when the image is edited sadRGB color-space than what there are when the image is edited in an accurate gamma 2.2 color-space. sadRGB has the gamut (primaries or phosphors set) named as the HDTV, there are no display systems that have these primaries, the vast majority of CRT monitor have primaries that have the name Trinitron (this does not have anything to do with the mask technology that also is called as Trinitron). Due to this gamut issue sadRGB images that you see on the Web often have slight reddish or magentaish cast. So, sadRGB really is not good at all for Web publishing.
So in practical terms how do I use it? With RGB images
that I want to convert or work on in Black & White?
Photoshop is automatically using the current RGB and grayscale working-space profiles. So when you convert from RGB to Grayscale then it is converted from the current space that the image has to your grayscale working-space profile. Also the individual channel views are shown using that gray profile.
If you work in an other RGB working space than the sadRGB then it is sensible (necessary) to change also the Gray working space profile,
e.g. if you work in a linear RGB working space then open the
Edit/ColorSettings dialog, there open the Working Spaces Gray dropdown, select the second item from the top of that list (Custom Gamma) and enter the value 1.0 to that dialog. If you insist to work in a non-linear RGB working-space like the adobeRGB then do the above but enter the value 2.2 to the custom gamma dialog.
Timo Autiokari
http://www.aim-dtp.net/